This invention relates to an electrophotographic plate for electrophotographic printing plate making.
Hitherto, there have been known light sensitive materials for lithographic printing plate, which use light sensitive resins or silver halide light sensitive materials. However, although lithographic printing plates made using light sensitive resins are high in printing endurance and excellent in image reproducibility, they have defects that sensitivity is low and sensitive wavelength region is on the short wavelength side. Therefore, there have been problems that they are not suitable for direct plate making from originals and besides light sources are limited for laser scanning.
Furthermore, printing plates according to diffusion transfer development or hardening development of silver halide light sensitive materials can be produced with satisfactory sensitivity, but they also have the defects of poor printing endurance and high cost for one plate.
As materials for direct plate making by electrophotography, there have been known those of zinc oxide-resin dispersion type. In case of printing plate of this type, charged latent image is formed on a zinc oxide-resin dispersion coating film by exposure to light in an imagewise manner and is developed with a toner and finally non-image area is rendered hydrophilic. The printing plate made in this way is damaged in its surface hydrophilic layer due to rupture of layer caused by mechanical pressure during printing or by permeation of damping solution into light sensitive layer (conducted layer) and so such printing plate is also inferior in printing endurance like printing plate of silver halide light sensitive type.
On the other hand, for printing plates according to electrophotography, there have been known light sensitive materials which comprise a conductive support sandblasted and rendered hydrophilic and an electrophotographic light sensitive layer of an alkali-soluble resin provided on the support. Plate making is carried out by charging and exposing the light sensitive layer, developing the latent image with a toner and removing the light sensitive layer of non-image area by an alkali solution for dissolving-out of the layer. As a result, there are formed non-image areas of hydrophilic surface of aluminum oxide and image areas of electrophotographic light sensitive layer which are covered with toner.
Thus, printing plates according to electrophotography have the advantages that they are superior in printing endurance and further, sensitive wavelength region can be optionally determined by selection of organic photoconductive substances. However, these printing plates are made through the step of dissolving-out of the non-image area with an alkali and hence the image reproducibility delicately depends on the balance between alkali-solubility of resin and resin-dissolvability of alkali.
Especially important problem is that dissolving-out property of non-image area varies at interface between light sensitive layer and support and dissolving-out property of light sensitive layer at that portion deteriorates to leave residues and when dissolving-out property of light sensitive layer is increased for removing the residues, printing endurance becomes inferior.